Conventionally, the rotational speed of rotating components in exhaust-gas turbochargers is determined using the RADAR principle. For example, German Patent Application No. DE 10 2012 200 261 A1 describes a set-up for measuring a rotational speed of a turbocharger, the set-up having a rotational speed sensor unit, which includes a radar transceiver for emitting and receiving radar waves. This receives the radar waves reflected by a machine element and makes them available as a measuring signal for an evaluation unit. The rotational speed of the turbocharger is ascertained with the aid of the evaluation unit.